Ever increasing population results in a continuously increasing amount of solid waste. It also places stress on water supplies since potable water is used to flush much of human solid waste to treatment plants and sometimes directly to bodies of water used for human consumption. Pollution control authorities such as the Environmental Protection Agency, require that sewage be treated in several stages before being released into lakes, rivers, or the ocean. In 1985, Public Operated Treatment Works (POTW) were required to upgrade water treatment facilities to include secondary treatment for plants with ocean discharge and tertiary treatment for other plants. The end result of these regulations is improved water quality and increased volumes of sewage sludge. Sewage is now disposed of in land fills, surface sites, incineration or application to land. Land fill and surface sites are rapidly filling. Incineration requires the use of expensive fuel and contributes to air pollution. A natural use for sewage sludge and other solid, animal waste products such as residuals of waste water treatment, septages and animal manure would appear to be application to land in agricultural production due to the organic and mineral components of the waste.
However, feces containing solid waste tends to have a high pathogen content. If the sludge is not treated to reduce the pathogen content, land receiving application of the waste can not be used for animal grazing or food crop production for 5 years after the last application of solid waste. Furthermore, solid waste may contain heavy metal ions which may be hazardous to animals or could accumulate in the soil and render it unsuitable for agriculture.
Regulations have been promulgated by the Environmental Protection Agency establishing criteria and conditions for the reuse of these materials. Use limitation criteria are based on deleterious constituents such as heavy metal and pathogenic organisms. Concurrent with reuse guideline development has been the reduction in the number of disposal sites willing and capable of accepting sludges, septages, manures and residuals. Similarly, manures are being increasingly scrutinized for the impact from storage facilities on ground and surface waters. The net effect of these regulations has been dramatic increase in the cost for treatment and disposal of sludges, manures and septages.
Several Processes to Significantly Reduce Pathogens (PSRP) have been developed that reduce both pathogen levels and the attractiveness of sludges to disease vectors. The processes effectively reduce pathogenic viruses and bacteria by about 90%. The PSRP process that have been recognized are aerobic digestion, anaerobic digestion, lime stabilization, air drying and composting.
Aerobic digestion involves biochemical oxidation of sludge in an open or closed aerobic tank and can be practiced in a batch or continuous mode. The digestion requires 40 to 60 days residence time at temperatures from 15 to 20 degrees Celsius. Anaerobic digestion is conducted in the absence of air. Even with added heat the process still requires 15 days to digest the waste.
In air drying the wet sludge is generally applied to sand and/or gravel beds to a depth of up to about 9 inches. To be considered a PSRP the sludge must be air dried for at least 3 months. Lime stabilization involves adding lime to sludge in a sufficient quantity to produce a pH of 12 after 2 hours. The treatment period is short. However, lime is expensive and the pathogens can regrow if the pH drops below 11. Composting to meet PSRP conditions requires treatment for at least 5 days at 40 degrees Celsius with 4 hours at a temperature of at least 55 degrees Celsius.
The PSRP processes can be combined with other processes to further reduce the pathogen to a level below the detection limit. Some of the same processes discussed as PSRP processes can qualify as a PFRP (Process to Further Reduce Pathogens) if operated at high temperature.
Treatment processes demonstrated to be effective in reducing pathogen content of waste sludges, septage and waste water residuals have been identified and defined by the EPA. Regulations (40 CFR parts 257 and 503) provide necessary criteria for sludge product treatment and usage. States have the option to either adopt federal standards or justify and adopt other equivalent or more restrictive use limitations.
Methods generally approved as Process to Further Reduce Pathogens (PFRP), the most substantial pathogenic organism reduction option, can be summarized as thermal treatments from external heat sources such as incinerators and dryers (pressurized or at ambient atmospheric pressures). Heat treatment for pathogenic reductions also include thermophilic decomposition (composting) and thermophilic aerobic digestion which utilize temperature increases from biologic activity to reduce pathogenic organisms to PFRP standards. Non-thermal processes for PFRP treatments include chemical disinfection and radiation of sludge solids (electron, gamma ray, ultraviolet).
The reuse of sludge is also limited by concerns other than pathogenic content. End product qualities and raw waste constituents (heavy metals) have frequently affected the ability to use the end product in certain environments. While not a significant problem in sewage sludges, soluble arsenic compounds may be of concern in specialized situations.
All of the approved processes involve the use of substantial amounts of land or equipment to hold large bodies of waste for long holding periods or the application of heat from external sources to reduce the holding time during treatment.
Other processes for treatment of waste can be utilized if the use proves that the process results in effective removal of pathogens from the waste.